The present invention relates to marine seismic prospecting. More particularly, it has as its object an improved method for marine seismic prospecting, in which the interpretation of data, that is the results recorded after the shots, is improved through the elimination of parasite data of the "water bottom multiple" type or the elimination of "peg legs", as referred to in the art. These terms will be defined later in the present specification.
A widely used known research technique for oil or gas consists of carrying out seismic prospecting of the subsoil. In land seismic measurements, from a point on the surface of the ground, seismic waves are emitted, by creating for example the explosion of a powerful charge. Detectors which are called geophones, placed at various points of the surface, collect the acoustic waves which are reflected by the interfaces or "horizons" between geological layers. These detectors convert the reflected waves into electrical signals, and are provided in sufficient number and arranged such that the recorded signals enable the configuration of the geological layers to be reconstituted. In practice, the signal recorded by a geophone has an amplitude which varies constantly as a function of time. The peaks recorded generally correspond to horizons between layers.
In reality, the interpretation of data from a shot is much more complex, because, on the one hand, the waves can traverse a first interface between layers and be reflected on the next interface, then on the first interface and so on before reaching the geophone; on the other hand, the amplitude of the recordings decreases very rapidly depending on the time. The recordings therefore contain peaks corresponding to multiple reflections which it is appropriate to eliminate in order to correctly reconstitute the configuration of the subsoil.
A similar technique is used for marine seismic prospecting, with the shot of an explosive charge being carried out in this case at a few meters below the surface of the water and the receivers or "hydrophones" themselves being arranged at the same depth. In this method of prospecting, the disadvantages indicated above are moreover amplified by the fact that the sea bed is highly reflective, as is the water-air interface. A seismic wave will therefore be reflected successively on these two interfaces, giving rise, on the hydrophones, to parasite signals corresponding to the multiple reflections of the water layer.
The multiple reflections can be of different types and the present invention relates more particularly to the multiple reflections of the sea bed, for which the seismic wave does not penetrate into the geological formations but is reflected on the sea bed, and to the "peg legs" for which the wave penetrates into the underground formations and then is reflected on an interface before again traversing the water and rebounding into this layer of water.
As can be seen in FIG. 1, an explosion of a charge at a point A situated close to the surface S of the water can give rise to a wave 0 which is reflected against the sea bed F, then against the air-water interface at the surface S and again against the sea bed F, before reaching the hydrophone arranged at the point H situated close to the surface of the water. The signal which the receiver will record will therefore be representative of neither the sea bed nor the interface of the two geological layers, but of a multiple reflection of the sea bed.
In FIG. 2, the wave 0 traverses the bed F to penetrate into a subjacent geological formation, then is reflected on the interface I separating that formation from the next layer. The reflected wave traverses the bed F to return into the layer of water. It is again reflected against the surface S, then against the bed F, before reaching the hydrophone H, which records a signal corresponding to the path followed by the wave 0. This path has the profile represented in FIG. 2 and, because of its asymmetrical form, it is familiarly known in the art by the expression "peg leg", or "wooden leg".
Coherent signals, representative of sea bed multiples and peg legs are consequently found on the recordings made by the geophones, but they are completely devoid of interest for knowledge of the subsoil. Moreover, they can be superposed on primary reflections and, therefore, conceal useful information. It is therefore appropriate to eliminate them from the data obtained following shots, in order to obtain a representative recording of the geological formations alone.